These days, surface plasmon resonance (to be referred to as SPR) measuring devices have been studied as a biosensor using light (see, e.g., Japanese Patent Laid-Open No. 2001-194298, Japanese Patent No. 3356213, and reference: “‘Integrated Fluid Handling System for Biomolecular Interaction Analysis’, Analytical Chemistry, 1991, Vol. 63, No. 20, pp. 2338-2345”). The SPR measuring device uses a measurement sample cell obtained by immobilizing a substance film to be measured, such as an antibody, to a metal thin film made of gold, silver, or the like. The sample cell is irradiated with light from a surface opposite to the antibody. The SPR measuring device measures an incident angle at which the evanescent wave of the light and the surface plasmon wave resonate with each other.
FIG. 34 is a block diagram showing the schematic arrangement of a conventional SPR measuring device. The SPR measuring device includes a prism 1001, light source 1002, polarizing plate 1003, condenser lens 1004, and CCD camera 1005.
When light emitted by the light source 1002 for monochromatic light reaches the polarizing plate 1003, only p-polarized light passes. The p-polarized light is condensed by the condenser lens 1004 and enters the semi-columnar or hemispherical prism 1001. A sample cell 1000 is set on the upper surface of the prism 1001. The p-polarized light enters the sample cell 1000 from a surface opposite to one on which a substance film to be measured such as an antibody is immobilized. The p-polarized light enters the sample cell 1000 at the incident angle θ via the prism 1001. The CCD camera 1005 detects an intensity change of the light reflected by the sample cell 1000.
Light emitted by the light source 1002 becomes an evanescent wave at the boundary between the prism 1001 and the metal thin film of the sample cell 1000. A surface plasmon wave is generated on the surface of the metal thin film. At the incident angle θ at which the wave numbers of the evanescent wave and surface plasmon wave match each other, the evanescent wave is used for excitation of the surface plasmon wave, decreasing the quantity of light measured as reflected light. At this time, the CCD camera 1005 measures the intensity of the reflected light, observing a decrease in reflectance at the incident angle at which the evanescent wave and surface plasmon wave resonate with each other, as shown in FIG. 35. On an incident angle-reflectance curve representing the relationship between the incident angle and the reflectance, a low-reflectance valley appears near the incident angle at which the evanescent wave and surface plasmon wave resonate with each other.
The angle at which the evanescent wave and surface plasmon wave resonate with each other depends on the refractive index of a substance film to be measured in contact with the metal thin film of the sample cell 1000. When the substance film to be measured such as an antibody is immobilized on the metal thin film, the refractive index of the antibody changes owing to antigenic binding, and the angle at which the valley appears slightly changes. By measuring this change, the substance film to be measured can be quantified.